Time machine external disk size

[Rowf]

I've been reading about time machine and thinking it's time to get grips with a back up.
The info I've read states that the external hard drive that time machine backs up to should be bigger than the disk that is being backed up.
What I'm not clear on is this:
I have a 250gb disk on my macbook but am only using around 20gb of it so far, so does time machine just back up the file system and folders in use? i:e, the 20gb, in which case I can't see that the external hard drive being smaller would be a problem initially, maybe in time as more is added to the drive, but not for now?
If so, could I get away with using a 160gb external hard drive?
I have a 'spare' one that I can use if I can get away with it so I don't want to buy a new one if it's not necessary.
Thanks.

[richard.mac]

say you are using half of your hard drive and Time Machine does an initial backup of 125GB, then Time Machine only has another 125GB to do incremental backups with. so, if you edit existing large files like media files they will be continually be incrementally backed up and the backup hard drive will eventually get full.

thats why its good to have a backup drive double the size of your boot drive. if large files arent incrementally backed up then its ok to have a backup drive the same size as your boot drive. eventually though, your backup drive will get full quicker than your boot drive because of the incremental backup nature of Time Machine.

[Rowf]

Thanks for your reply, so I'll be better to be on the safe side and pick up, say, a 500gb external drive dedicated to the time machine backup?
Apologies about seeming a bit docile about this but on windows I've usually just created an image backup every month or so, 'real time' backups are a very new to me.
Further to this, if the backup drive isn't plugged in for a few days does it then catch up on changes that have been made since the last time that it was used?

[richard.mac]

same size backup drive is fine if youre doing a clone/image (Carbon Copy Cloner, SuperDuper mac apps) but recommend >1.5x for Time Machine incremental backups.

and yep Time Machine backs up all the changed files since the backup drive was last connected. not sure if its instant, but will happen after an hour has past.

[Rowf]

Thanks for the info and the quick reply, just what I needed to know.

Cheers.

[McRCN]

You might find this site helpful.

http://web.me.com/pondini/Time_Machine/1.html

[Rowf]

Thanks for the link, the mystery of time machine and backing up on the macbook is now being unveiled.

While I'm here a quick basic forum question.
How do I mark a thread as solved?